The present invention generally relates to interferometry, and more particularly to a quadrature phase shift interferometer.
A form of an information storage and retrieval device is a hard disc drive [hereinafter xe2x80x9cdisc drivexe2x80x9d]. A disc drive is conventionally used for information storage and retrieval with computers, data recorders, redundant arrays of independent discs (RAIDs), multi-media recorders, and the like. A disc drive comprises one or more disc media.
Each disc medium comprises a substrate upon which materials are deposited to provide a magnetically sensitive surface. In forming a disc medium, a substrate is ground or polished, conventionally by chemical-mechanical or mechanical polishing, to provide a substantially planar surface. Layers of materials are substantially uniformly deposited on this substantially planar surface to provide magnetic properties for writing to and reading from the disc media.
However, defects, such as pits, voids, particles, bumps, and scratches, among others may arise on a disc medium surface. These defects need to be detected and characterized. Interferometers can be used to detect and characterize such defects. Displacement demodulation is conventionally done by counting fringes or demodulating phase. However, such interferometers have a relatively large number of optical-components, as may be seen in U.S. Pat. No. 5,999,261.
Accordingly, it would be desirable to provide a method and apparatus for interferometry with a relatively small number of optical components to reduce optical path variation due to air turbulence, as well as to reduce energy loss due to ghost images reflected from optical component surfaces. Moreover, by providing an interferometer with a relatively small number of optical components, stability and immunity to thermal expansion and environmental vibration may be enhanced while cost is reduced.
The present invention provides method and apparatus for inspecting a disc medium surface. More particularly, a quadrature phase shift interferometer designed to provide an optical, non-contact testing method for inspecting a disc medium surface, or other surface, is described. Defects are detected and characterized by out-of-plane displacement. The interferometer described is able to measure out-of-plane displacements with nanometer resolution with frequency response in a range of DC to hundreds of mega hertz depending on detector rise time. Such an interferometer comprises fewer optical components as compared with prior art interferometers of at least equivalent capability.
An aspect of the present invention is an interferometer for disc surface inspection. The interferometer comprises a laser configured to provide a linearly polarized laser beam, and a variable ratio beam splitter positioned to receive the linearly polarized laser beam and configured to split the linearly polarized laser beam into a reference beam and an object beam. The reference beam and the object beam are polarized beams with polarizations orthogonal to one another. A mirror is positioned to reflect the reference beam back toward the variable ratio beam splitter to provide a reflected reference beam. The disc surface is positioned to reflect of the object beam back toward the variable ratio beam splitter to provide a reflected object beam. The variable ratio beam splitter is positioned to receive the reflected reference beam and the reflected object beam, and configured to combine the reflected reference beam and the reflected object beam to provide a combinative beam. A nonpolarizing beam splitter is positioned to receive the combinative beam and configured to split the combinative beam into a first output beam and a second output beam. An adjustable quarter-wave plate is positioned to receive the first output beam and configured to introduce a phase shift between the reflected object beam portion of the first output beam and the reflected reference beam portion of the first output beam to provide a phase-shifted output beam. A first polarizer is positioned to receive the phase-shifted output beam and configured to assemble the phase-shifted output beam along a predetermined direction to provide[d] a first assembled beam. A second polarizer is positioned to receive the second output beam and configured to assemble the second output beam along the predetermined direction to provide a second assembled beam. A first optical detector is positioned to receive the first assembled beam and configured to provide a first voltage proportional to change in intensity due to interference of the reflected object beam portion and the reflected reference beam portion of the first assembled beam, and a second optical detector is positioned to receive the second assembled beam and configured to provide a second voltage proportional to change in intensity due to interference of the reflected object beam portion and the reflected reference beam portion of the second assembled beam.
An aspect of the present invention is a method for media surface inspection. The method comprises of providing a linearly polarized laser beam. The linearly polarized laser beam is split into a reference beam and an object beam based on polarization. The reference beam is reflected from a mirrored surface to provide a reflected reference beam. The object beam is reflected from the medium surface to provide a reflected object beam. The reflected reference beam and the reflected object beam are combined to provide a combinative beam. The combinative beam is split into a first output beam and a second output beam based on amplitude. A phase-shift is introduced between the reflected object beam portion and the reflected reference beam portion of the first output beam to provide a phase-shifted output beam. The phase-shifted output beam is assembled at an angle with respect to direction of polarization to provide a first assembled beam. The second output beam is assembled at the angle to provide a second assembled beam. Fringes of the first assembled beam are detected to provide a first voltage, and fringes of the second assembled beam are detected to provide a second voltage.
Another aspect of the present invention is a method for inspection of a disc medium surface. The method comprises determining a first intensity for a first beam voltage, determining a second intensity for a second beam voltage, determining a first phase angle for the first intensity, determining a second phase angle for the second intensity, adding positive and negative values of a constant to the first phase angle and the second phase angle in response to slope direction of the first phase angle and the second phase angle to provide a phase function, and determining displacement caused by variations in the disc media surface.